Universal Serial Bus, or USB, is a bus, standard widely used in personal computers, servers, cameras, DVD players, mobile phones and other applications. A USB system has an asymmetric design, typically consisting of a host, a single or a plurality of downstream USB ports, and one or more peripheral devices connected in a tiered-star topology. Additional USB hubs may be included in the tiers, allowing branching into a tree structure with up to five tier levels.
The USB VBUS signal is part of the USB specification. Specifically, the VBUS signal is one of the four signals required by USB. The other three USB signals are a ground (GND) signal, and the positive and negative wires of a differential data signal (labeled as D+ and D−). VBUS is required by both bus-powered and self-powered Hubs and peripheral devices. For bus-powered peripheral devices, VBUS is the power line provided by a USB host, providing the primary power for such devices. For self-powered peripheral devices, VBUS signals a “powered state” required by the USB specification. The VBUS signal may also be used by self-powered peripheral devices to maintain USB interface functionality when the remainder of the self-powered peripheral device is powered down.
In order to be in compliance with USB, the VBUS signal must maintain specific voltage and current levels. According to the USB specification, the VBUS signal voltage may vary from 4.35 to 5.25 volts with respect to ground. However, if a USB host supplies voltage at or near the 4.35 volt lower limit, there is a risk of device resets or poor reliability, so bus attenuation must be considered when designing a USB network.
The USB specifications are primarily concerned with cable connectivity, not PCA-networked systems. PCA (printed circuit assembly) routed signals have different attenuation issues than a cable routed signal. A signal routed across PCA-to-PCA connectors can accumulate significant attenuation. Consequently, a VBUS signal routed across PCA-networked systems with multiple PCA-to-PCA connectors can lose its voltage margin and fall below the 4.35 volt lower limit. Accordingly, such PCA-networked systems provide a significant hurdle for USB VBUS signal propagation.
VBUS signal propagation is less problematic in cable networks. Therefore, connecting PCAs together with cables would seem a solution to the VBUS signal propagation issue. However, it is often desirable to minimize the number of cable connections by routing directly from PCA-to-PCA. Cable connectors take up significantly more area than a routed trace on a PCA. Moreover, cable connectors do not survive shock and vibe testing as well as routed traces. Furthermore, systems with large numbers of cables can confuse end-users and impose additional mechanical cable management requirements and PCA bulkhead space.
USB repeaters and hubs (i.e., an electrical interface between a USB host and USB device that provides connectivity management and repeater functionality) can be used to address the VBUS signal propagation issue across PCAs. However, solutions utilizing repeaters or hubs have significant limitations. For example, such a solution requires the addition of specialized parts (i.e., the repeaters and hubs) to each PCA. This adds expense and may cause problems because of space and size limitations. Additionally, midplane PCA and backplane PCA requirements often preclude the use of active parts, so repeaters or hubs cannot be used on many midplanes and backplanes. Likewise, repeaters and hubs require a specific voltage rail which may not be readily available on all PCAs without adding new power circuits. This also adds expense and takes up space is often not practical for PCAs with small or limited areas or for PCAs that must be of a minimal cost.